Showing papers by "Atomic Energy of Canada Limited published in 2006"

Capacitance Sensors for Void-Fraction Measurements and Flow-Pattern Identification in Air–Oil Two-Phase Flow

Abstract: The design methodology of capacitance sensors for void-fraction measurement in adiabatic two-phase flow systems is presented in this paper. The effect of design parameters on the capacitance output has been theoretically and experimentally investigated for two types of sensor configurations: concave and ring types. Experiments were performed using air-oil two-phase flow to determine the signal-to-noise ratio, sensitivity, and time response of the capacitance sensors. The results show that the ring-type sensors are more sensitive to the void-fraction signal than the concave type for the same spatial resolution. The predictions from the theoretical model for the ring-type sensors are in better agreement with the experimental results than for the concave type. The mean value, time trace, power spectral density (PSD), and the probability density function (PDF) of the void-fraction signal from the capacitance sensors are used to objectively identify the flow pattern. The method was validated using high-speed video images of the flow and comparing the results to those from the signal analysis

Micromechanical characterisation of TRIP-assisted multiphase steels by in situ neutron diffraction

Abstract: The flow behaviour of the constitutive phases in multiphase steels, possibly exhibiting a mechanically-induced phase transformation ( TRIP effect), is investigated using neutron diffraction conducted during uniaxial tensile loading. The BCC and FCC lattice strains of several specimens containing different amounts ferrite, bainite, martensite and metastable retained austenite are measured along elastic and plastic deformation. The validity of the measurements, as well as the strengthening resulting from the TRIP effect, are evaluated on the basis of overall mechanical equilibrium.

Low Doses of Radiation are Protective In Vitro and In Vivo: Evolutionary Origins

Abstract: Research reports using cells from bacteria, yeast, alga, nematodes, fish, plants, insects, amphibians, birds and mammals, including wild deer, rodents or humans show non-linear radio-adaptive processes in response to low doses of low LET radiation. Low doses increased cellular DNA double-strand break repair capacity, reduced the risk of cell death, reduced radiation or chemically-induced chromosomal aberrations and mutations, and reduced spontaneous or radiation-induced malignant transformation in vitro. In animals, a single low, whole body dose of low LET radiation, increased cancer latency and restored a portion of the life that would have been lost due to either spontaneous or radiation-induced cancer in the absence of the low dose. In genetically normal fetal mice, a prior low dose protected against radiation-induced birth defects. In genetically normal adultmale mice, a low dose prior to a high dose protected the offspring of the mice from heritable mutations produced by the large dose. The results show that low doses of low-LET radiation induce protective effects and that these induced responses have been tightly conserved throughout evolution, suggesting that they are basic responses critical to life. The results also argue strongly that the assumption of a linear increase in risk with increasing dose in humans is unlikely to be correct, and that low doses actually reduce risk.

Determination of femtogram quantities of 239Pu and 240Pu in bioassay samples by thermal ionization mass spectrometry

Abstract: A thermal ionization mass spectrometry (TIMS) method is described for the determination of ultra-trace levels of plutonium isotopes in human urine samples. The method has been validated through the analysis of artificial urine samples spiked with known amounts of 239Pu ranging from 2.5 fg to 50 fg (6-115mBq). A slight positive bias of 1.7%-2.7% was determined, with a relative precision of 2.2% at 50 fg, increasing to 2.7% for 5-25 fg 239Pu. The detection limit of the method was 0.53 fg (1.2mBq) 239Pu, and the instrumental detection limit was at least 0.1 fg. The determination of the isotopic signature of the sample with 239Pu, 240Pu, and 241Pu amounts of several femtograms is possible, and was demonstrated with the determination of the 240 to 239 ratio in an inter-laboratory sample comparison. The method is relatively free from interferences, 95% of sample preparations were acceptable both in terms of chemical recovery and lack of isobaric interference. The isotopic abundance of the 242Pu SRM 4334E of the National Institute of Standards and Technology (NIST) was also determined by TIMS and was found to be 99.99967 atom% 242Pu.

Future Hydrogen Production Using Nuclear Reactors

Abstract: The potential of hydrogen to replace fossil fuels presents a significant opportunity for reducing greenhouse gas (GHG) emissions, especially when hydrogen is produced by "water-splitting", instead of hydrocarbon processing. "Water- splitting" by energy derived from nuclear sources is a preferred method for "carbon-free" production of hydrogen on a large scale. Researchers around the world are pursuing two new ways of water-splitting - thermochemical cycles and high- temperature electrolysis (HTE), using thermal energy from the future generation of higher temperature reactors. Both these methods, when coupled with a high-temperature nuclear reactor, could have efficiencies in the range of 50-60% compared to <30% for conventional electrolysis - currently the only existing method of producing hydrogen without co-product CO2. Research is underway at Atomic Energy of Canada Limited (AECL) on the development of the next generation of advanced CANDUreg concepts that include the Supercritical Water-Cooled Reactor (SCWR). The SCWR would use supercritical water as the coolant with a nominal outlet temperature of up to 6251 and could deliver heat at ges550degC for hydrogen production. AECL is currently evaluating various thermochemical cycles and high-temperature electrolysis for matching with the temperature capability of the SCWR and ACR-1000reg.

The Srk Inventory: A Tool for Structuring and Capturing a Worker Competencies Analysis

Abstract: Worker Competencies Analysis (WCA) is the fifth and final phase of the Cognitive Work Analysis (CWA) framework. Unlike the earlier four phases, there is a dearth of published work illustrating how ...

Nuclear Fission Fuel is Inexhaustible

Abstract: Nuclear fission energy is as inexhaustible as those energies usually termed ldquorenewablerdquo, such as hydro, wind, solar, and biomass. But, unlike the sum of these energies, nuclear fission energy has sufficient capacity to replace fossil fuels as they become scarce. Replacement of the current thermal variety of nuclear fission reactors with nuclear fission fast reactors, which are 100 times more fuel efficient, can dramatically extend nuclear fuel reserves. The contribution of uranium price to the cost of electricity generated by fast reactors, even if its price were the same as that of gold at US$14,000/kg, would be US$0.003/kWh of electricity generated. At that price, economically viable uranium reserves would be, for all practical purposes, inexhaustible. Uranium could power the world as far into the future as we are today from the dawn of civilization-more than 10,000 years ago. Fast reactors have distinct advantages in siting of plants, product transport and management of waste.

Scattering from laterally heterogeneous vesicles. I. Model-independent analysis

Abstract: This is the first of a series of papers considering the scattering from laterally heterogeneous vesicles. Here, it is shown that contrast variation studies on unilamellar vesicles can be analyzed in a model-independent manner to detect lateral segregation in model membranes. In particular, it is demonstrated that the Porod invariant, Q = \textstyle\int q^{2}I (q )\,{\rm d}q, is related to the scattering length density contrast between compositionally distinct regions in a heterogeneous membrane. The formation of domains and the concomitant identification of phase boundaries as a function of either membrane composition or temperature can therefore be detected in the changes taking place in Q.

Development of a heterogeneous microstructurally based finite element model for the prediction of forming limit diagram for sheet material

Abstract: A heterogeneous finite element model with randomly distributed inhomogeneities has been developed for the determination of the forming limit diagram (FLD) for thin aluminum sheet material based on the prediction of localized necking. The strength difference between the inhomogeneities and the matrix is ascertained either from the fluctuation of the experimental stress-strain curve or from a micromechanical analysis that uses a representative particle field. By changing the specimen geometry and friction conditions, different stress states (or strain paths) are achieved. A plot of the critical Oyane fracture parameter is used to identify the limit strain state. Also, a plot of equivalent plastic strain rate is used to distinguish the boundary of intense shear bands and hence to identify where to take the measurement point. Both a plane stress model and a three-dimensional (3-D) model are adopted to predict the shear banding phenomenon and hence the FLD. The predicted FLD agrees well with the measurements from a recent round robin experimental FLD involving several independent research laboratories. The Taguchi method is applied to assess how the various parameters involved in the heterogeneous model affect the calculated forming limit strain.

A Fuel Channel Design for CANDU-SCWR

Abstract: The CANDU® -Supercritical Water Reactor (CANDU-SCWR) is one of the six reactor concepts being considered by the Generation-IV International Forum (GIF) for international collaborative R&D. With SCW coolant, the thermodynamic efficiency is increased to over 40%. The CANDU-SCWR is moderated using heavy water, and it has fuel bundles residing inside horizontal pressure tubes, similar to the current CANDU design. The coolant, however, is light water at 25 MPa, with an inlet temperature of 350°C and an outlet temperature of 625°C. Because of the high temperature and high pressure of the coolant, the standard CANDU pressure tube design cannot be used. This paper presents one of the insulated pressure tube designs being considered for the CANDU-SCWR fuel channels. Unlike current CANDU reactors, the proposed CANDU-SCWR fuel channel does not use calandria tubes to separate the pressure tubes from the moderator. Each pressure tube is in direct contact with the moderator, which operates at an average temperature of about 80°C. The pressure tube is thermally insulated from the hot coolant by a porous ceramic insulator. A perforated metal liner protects the insulator from being damaged by the fuel bundles and erosion by the coolant. The coolant pressure is transmitted through the perforated metal liner and insulator and applied directly to the relatively cold pressure tube. The material selection for each fuel channel component depends on its function. The fuel sheaths and the perforated liner must have high corrosion resistance in SCW, although their resident times are significantly different. The insulator must have high thermal resistance and corrosion resistance in SCW, plus sufficient strength to bear the weight of the fuel bundles without significant thickness reduction during its design life. The pressure tube is the pressure boundary material, so it must have high strength to contain the coolant. One common requirement for all in-core fuel channel components is that they should be as neutron transparent as possible. The irradiation deformation of all these components must also be considered in their design. This paper presents the design of this fuel channel, reviews existing data for materials, indicates where more data are required, and summarizes our plans to obtain these data.Copyright © 2006 by Atomic Energy of Canada, Ltd.

CFD Application to Flow-Accelerated Corrosion in Feeder Bends

Abstract: Feeder piping in CANDU® plants experiences a thinning degradation mechanism called Flow-Accelerated Corrosion (FAC). The piping is made of carbon steel and has high water flow speeds. Although the water chemistry is highly alkaline with room-temperature pH in a range of 10.0–10.5, the piping has FAC rates exceeding 0.1 mm/year in some locations, e.g., in bends. One of the most important parameters affecting the FAC rate is the mass transfer coefficient for convective mass transport of ferrous ions. The ions are created at the pipe wall as a result of corrosion, diffuse through the oxide layer, and are transported from the oxide-layer/water interface to the bulk water by mass transport. Consequently, the local flow characteristics contribute to the highly turbulent convective mass transfer. Plant data and laboratory experiments indicate that the mass transfer step dominates FAC under feeder conditions. In this study, the flow and mass transfer in a feeder bend under operating conditions were simulated using the Fluent™ computer code. Because the flow speed is very high, with the Reynolds numbers in a range of several millions, and because the geometry is complex, experiments in a 1:1 scale were conducted with the main objective to validate flow simulations. The experiments measured pressure at several key locations and visualized the flow. The flow and mass transfer models were validated using available friction-factor and mass transfer correlations and literature experiments on mass transfer in a bend. The validation showed that the turbulence model that best predicts the experiments is the realizable k-e model. Other two-equation turbulence models, as well as one-equation models and Reynolds stress models were tried. The near-wall treatment used the non-equilibrium wall functions. The wall functions were modified for surface roughness when necessary. A comparison of the local mass transfer coefficient with measured FAC rate in plant specimens shows very good agreement. Visualization experiments indicate secondary flows in the bends. No boundary layer separation was observed in experiments or in simulations.Copyright © 2006 by AECL

The Effects of Dry Density and Porewater Salinity on the Physical and Microbiological Characteristics of Compacted 100% Bentonite

Abstract: This study examined the conditions required to suppress microbial activity in compacted bentonite, such that microbially influenced corrosion (MIC) of copper waste containers, surrounded by compacted bentonite in a future deep geologic repository, would become insignificant. Experiments were carried out to determine the effects of dry density and porewater salinity on swelling pressure, water activity (aw) and the culturable microbial community in compacted bentonite. A dry density ³ 1.6 g/cm3 ensures that aw is 2 MPa. Both conditions suppress microbial culturability below background levels (2.1 x 102 Colony-Forming Units/g) in as-purchased bentonite. Under such conditions, cells likely survive as dormant cells or inactive spores, which greatly reduces the possibility of significant MIC. Observations in natural clay-rich environments support these findings.

Quantification of microsegregation during rapid solidification of Al-Cu powders

Abstract: A new technique is introduced to quantify microsegregation during rapid solidification. The quantification involves calculation of the average solute solubility in the primary phase during solidification of an Al-Cu binary alloy. The calculation is based on using volume percent eutectic and weight percent of second phase (in the eutectic), which were obtained experimentally. Neutron diffraction experiments and stereology calculation on scanning electron microscope images were done on impulse atomized Al-Cu alloys of three compositions (nominal), 5 wt pct Cu, 10 wt pct Cu, and 17 wt pct Cu, atomized under N2 and He gas. Neutron diffraction experiments yielded weight percent CuAl2 data and stereology yielded volume percent eutectic data. These two data were first used to determine the weight percent eutectic. Using the weight percent eutectic and weight percent CuAl2 in mass and volume balance equations, the average solute solubility in the primary phase could be calculated. The experimental results of the amount of eutectic, tomography results from previous work, and results from the calculations suggest that the atomized droplets are in metastable state during the nucleation undercooling of the primary phase, and the effect of metastability propagates through to the eutectic formation stage. The metastable effect is more pronounced in alloys with higher solute composition.

Waveband MUX/DEMUX Using Concatenated Arrayed-Waveguide Gratings

Abstract: We propose a new waveband MUX/DEMUX that uses two concatenated cyclic AWGs. The device can accommodate multiple input fibers simultaneously and as a result, the device cost and size of a waveband cross-connect can be significantly reduced.

Chapter 4.4 Neutron techniques

Abstract: The first section of this chapter briefly describes the fission and spallation mechanisms for producing neutrons. The general properties of the slow neutron spectra that they produce are discussed and the methods of using beams from steady (fission reactor) sources and pulsed (accelerator-driven) sources are compared. The second section provides an overview of the different kinds of beam-definition devices. Resolution functions are then described in the third section. Scattering lengths for neutrons are tabulated in the fourth section, and scattering and absorption cross sections, isotope effects and correction for electromagnetic interactions are discussed. The fifth section gives tables of the coefficients in analytic approximations to the form factors used in the calculation of the cross sections for magnetic scattering of neutrons. The coefficients for atoms and ions in the 3d and 4d transition series are derived from wavefunctions obtained using Hartree–Fock theory and the coefficients for the rare-earth and actinide ions are obtained by fitting the analytic forms to published form factors calculated from Dirac–Fock wavefunctions. In the final section, the absorption cross sections and 1/e penetration depths of the elements are tabulated for neutrons of 1.8 A wavelength. Keywords: absorption; choppers; collimation; cross sections; filters; magnetic form factors; mirrors; monochromators; neutron diffraction; neutron scattering; neutron-beam definition; neutrons; resolution functions; scattering lengths; spin-orientation devices; Zeeman polarizers

Integrating Large-Scale Co-Generation of Hydrogen and Electricity from Wind and Nuclear Sources (Nuwind ©)

Abstract: This paper provides the global, strategic and market analyses that show, by actual example, the potential synergism between wind and nuclear energy systems By adopting a balanced portfolio of electricity and hydrogen production that is market driven, a generation mix of advanced nuclear designs with supplementary wind power provides a route towards a secure, sustainable and safe energy future Primary energy supply is on the threshold of a revolution as profound as utilization of carbon-based fuels by the Industrial Revolution Improved energy efficiency is insufficient to bring CO2 emissions under control since the new revolution must cope with rapid expansion of energy demand in the economies of the Developing World as they climb toward Developed World levels of wealth Three broad options exist for acceptable, primary energy supply: nuclear, renewables, and carbon with sequestration From all of these, energy will emerge increasingly as electricity and require some form of storage Hydrogen is ideal both for storage of electricity and as a transportation fuel producing minimal greenhouse gas emissions The required scale of new primary energy deployment is so large that choosing between the three supply technologies is pointless: one should identify ways in which they can most effectively be blended NuWindcopy is one such blend It addresses the problem of wind's short-term and seasonal intermittency and unpredictability The extent to which wind's variability can be absorbed as electricity depends on the mix of other generation capacity on a power grid but wind power can usually contribute only a fairly small percentage before the need for back-up capacity becomes burdensome This has led to the suggestion of converting wind-generated electricity into hydrogen by electrolysis However, simple analysis shows that wind's low average output inflates the cost of electrolysis equipment beyond economic competitiveness With NuWind, judicious application of time-of-day variability in the value of electricity subsidizes the cost of electrolytic equipment When the value of electricity is high, electricity is sold to the grid When the value of electricity to the grid is low, electricity from both nuclear and wind sources are converted into hydrogen using cells designed to handle varying current density If the wind is blowing, the current to the electrolysis cells is turned up Low-cost cavern storage for hydrogen accommodates seasonal swings in the wind component Using hourly electricity price data from Ontario and Alberta and typical hourly wind data, the economics of large-scale hydrogen production by NuWind are shown to be comparable with those of steam-methane reforming with sequestration Over a wide range of electricity to hydrogen ratios, the estimated cost of bulk, undistributed hydrogen production by Nu Wind is below the US DOE's target of 2000 $/tonne (all dollars quoted are US) All nuclear and aeolian generation is always available to the grid at times of high demand

Thermohydromechanical Modeling of a Full-Scale Tunnel Sealing Clay Bulkhead

Abstract: The Tunnel Sealing Experiment (TSX) was an international project developed by Canada, Japan, France, and the United States. The TSX consisted of a clay bulkhead and a concrete bulkhead. The two bulkheads were at opposite ends of a 12-m-long sand-filled chamber. The heating phase involved the circulation of heated water through the TSX chamber to raise the temperature of the sealing system. This was done to evaluate the influence of elevated temperature on the performance of the bulkheads. Numerical analyses to simulate the thermohydromechanical evolution of the clay bulkhead and the surrounding rock of the TSX were performed using MOTIF finite-element program and compared with measured data. This paper focuses on a preliminary interpretation of the results from the fully coupled thermohydromechanical modeling of the clay bulkhead and the adjacent rock during heating. Discussion of the issues that will affect the ability to model a field application is provided in the course of comparing simulated and measured results.

Hydrogen: The Fuel That Drill Bits Cannot Reach

Abstract: As realization grows of the damaging cumulative effects of CO{sub 2} on our biosphere, the prospect of substituting hydrogen for oil-based fuels attracts growing attention. Japan provides a leading example of remedial action with the expectation of five million fuel-cell-powered vehicles in operation by 2020. But where will the fuel for these and the rest of a 'Hydrogen Age' come from? The hydrogen market used to be straightforward: small-scale or high-purity markets were supplied relatively expensively by electrolysis; the other 95% was supplied much more cheaply by reforming hydrocarbons -- mostly using steam-methane reforming (SMR) and low-cost natural gas. The recent rise in the price of hydrocarbons -- natural gas as well as oil -- plus the need to sequester CO{sub 2} has disrupted this scenario. It seems likely that this is a permanent shift driven by growing demand for limited low-cost sources of fluid hydrocarbons. So the traditional SMR route to hydrogen will be in competition with reforming of heavier hydrocarbons (particularly coal and residual oils) as well as with electrolysis based on electricity produced from low-CO{sub 2}-emitting sources. By 2025, new high-temperature thermochemical or thermo-electrolytic sources based on high-temperature nuclear reactors could be in contention. This paper assessesmore » the economics of all these potential sources of hydrogen and their price sensitivities. It also considers their environmental footprints. Is hydrogen from 'clean coal' or other lower value hydrocarbons cost-effective if it is also CO{sub 2}-free? Is intermittent low-temperature electrolysis based on nuclear- and wind-produced electricity (NuWind{sup C}) the best way or does the hydrogen future belong to thermochemistry or thermo-electrolytic sources? How can one produce hydrogen to upgrade Canada's vast oil sands resources without the detraction of a large CO{sub 2} processing penalty? Fortunately for our planet, switching to hydrogen is no more than a technical challenge with a range of possible solutions but we need to make that point clearly to the political decision-takers and be able to provide assurance that the preferred solution will not be a source of new problems. (authors)« less

Internal friction peaks associated with the behaviour of hydrogen in Zr and Zr–2.5Nb

Abstract: The origins/mechanisms of internal friction peaks P 2 and P 3 in specimens of solid δ-hydride ZrH 1.5 , Zr–2.5Nb + 60 at.% H, Zr + 18 at.% H, etc. have been studied. Both peaks occur in a specimen of Zr + 18 at.% H, and the height of P 3 increases with the quench temperature but peak P 2 does not vary with quenching. However, in specimens of ZrH 1.5 and Zr–2.5Nb + 60 at.% H, only peak P 2 is observed, except for a small shoulder in ZrH 1.5 denoted as P′, which does not vary with quenching either, even though quenching reduces Young's modulus. The height of P 2 is proportional to the square of the H concentration. From this dependence, the mechanism of peak P 2 is proposed to be the stress-induced redistribution of H atom-pairs within the fcc lattice of δ-hydride. The height of P 3 depends on the content of quench-induced hydride particles. The relaxation mechanism responsible for peak P 3 is proposed to be stress-induced motion of H atoms across the interface between the hcp lattice of α-Zr and the fct (or fcc) lattice of the quench-induced hydrides.

Design and Commissioning of a Multivariable Control System for a Gas Turbine Engine Test Facility

Abstract: This paper describes the design and implementation of a digital setpoint tracking multivariable controller for the inlet air treatment system of a new aerospace engine high altitude test facility (ATF) which has recently become operational at the National Research Council's campus in Ottawa. The air that is supplied to the ATF test cell is conditioned and controlled by the Inlet Air Treatment System (IATS). The IATS provides control of air mass flow rate and temperature supplied to the test cell with the IATS exit maintained at a given altitude pressure. The low temperatures required are achieved by compressing and expanding the IATS supplied air with radial turbomachinery, rather than cooling the air with conventional refrigeration machines. The nature of the IATS process with its valves, compressors, turbine, and heat exchangers is that the control of mass flow and temperature are highly coupled with strong interactions between process inputs and outputs. The multivariable controller consists of a decoupling block and two independent PI controllers, which were designed for the main branches of the decoupled process. In order to circumvent the effect of cross coupling, the decoupling block was designed for steady state decoupling. Then the two PI controllers were designed for each of the two decoupled main paths of the process. The decoupling block causes a paralleling of the main branches and the decoupling of the original process. The commissioning results show that excellent setpoint tracking and disturbance rejection was achieved, under the action of the digital controller.

SCW Pressure-Channel Nuclear Reactors: Some Design Features and Concepts

Abstract: Concepts of nuclear reactors cooled with water at supercritical pressures were studied as early as the 1950's and 1960's in the USA and Russia. After a 30-year break, the idea of developing nuclear reactors cooled with supercritical water (SCW) became attractive again as the ultimate development path for water-cooling. The main objectives of using SCW in nuclear reactors are 1) to increase the thermal efficiency of modern nuclear power plants (NPPs) from 33 -- 35% to about 40 -- 45%, and 2) to decrease capital and operational costs and hence decrease electrical energy costs ({approx}$ 1000 US/kW). SCW NPPs will have much higher operating parameters compared to modern NPPs (pressure about 25 MPa and outlet temperature up to 625 deg. C), and a simplified flow circuit, in which steam generators, steam dryers, steam separators, etc., can be eliminated. Also, higher SCW temperatures allow direct thermo-chemical production of hydrogen at low cost, due to increased reaction rates. Pressure-channel SCW nuclear reactor concepts are being developed in Canada and Russia. Design features related to both channels and fuel bundles are discussed in this paper. Also, Russian experience with operating supercritical steam heaters at NPP is presented. The main conclusion is that developmentmore » of SCW pressure-channel nuclear reactors is feasible and significant benefits can be expected over other thermal energy systems. (authors)« less

The Value of Non-Carbon Power and Emissions Avoidance

Abstract: This paper discusses the role and benefits of avoiding GHG emissions, and of deploying and promoting "emissions free" energy technologies. The projected needs, market penetrations and transition strategies for non-carbon sources to stabilize atmospheric concentrations of GHGs were examined, including estimates of the benefits and impacts on future atmospheric concentrations, climate change and energy mix. As a result of this approach, the value of avoiding or reducing GHGs were are able to determined. Two values were defined: a monetary value and a social value.

Residual Stresses in CANDU Feeder Bends: Effect of Bend Radius

Abstract: Since 1997, sections of nine feeder pipes have been removed at the Point Lepreau Generating Station (PLGS) because of cracking. All PLGS feeder cracks are axial in orientation and are located in the feeder bends. In all cases, cracks were either at the inside surface of the bend flanks (approximately 60° from the intrados symmetry plane) or at the outside surface of the bend extrados. Root cause analyses indicated that the residual stress has a significant role in these failures. In a typical feeder, there are a number of bends and welds, which are potential locations of high residual stresses. To reduce inspection scope, or to identify the highest risk components, a relative ranking of crack susceptibility is needed. This can be achieved using the residual stress data of these components. This paper compares the measured residual stress data in tight-radius and large-radius CANDU feeder bends. It was found that residual stresses are significantly higher for the bends with small bend radius (r) over diameter (D) ratios (for example, r/D = 1.5) as compared to those bends with large r/D ratios (r/D > 4). The differences in the magnitude of residual stress are consistent with the measured and calculated level of cold work in the two types of bends. It was concluded that the likelihood of cracking in large-radius bends is significantly smaller than that in tight-radius bends.Copyright © 2006 by Atomic Energy of Canada Limited

The Human Bathtub: Safety and Risk Predictions Including the Dynamic Probability of Operator Errors

Abstract: Reactor safety and risk are dominated by the potential and major contribution for human error in the design, operation, control, management, regulation and maintenance of the plant, and hence to all accidents. Given the possibility of accidents and errors, now we need to determine the outcome (error) probability, or the chance of failure. Conventionally, reliability engineering is associated with the failure rate of components, or systems, or mechanisms, not of human beings in and interacting with a technological system. The probability of failure requires a prior knowledge of the total number of outcomes, which for any predictive purposes we do not know or have. Analysis of failure rates due to human error and the rate of learning allow a new determination of the dynamic human error rate in technological systems, consistent with and derived from the available world data. The basis for the analysis is the “learning hypothesis” that humans learn from experience, and consequently the accumulated experience defines the failure rate. A new “best” equation has been derived for the human error, outcome or failure rate, which allows for calculation and prediction of the probability of human error. We also provide comparisons to the empirical Weibull parameter fitting used in and by conventional reliability engineering and probabilistic safety analysis methods. These new analyses show that arbitrary Weibull fitting parameters and typical empirical hazard function techniques cannot be used to predict the dynamics of human errors and outcomes in the presence of learning. Comparisons of these new insights show agreement with human error data from the world’s commercial airlines, the two shuttle failures, and from nuclear plant operator actions and transient control behavior observed in transients in both plants and simulators. The results demonstrate that the human error probability (HEP) is dynamic, and that it may be predicted using the learning hypothesis and the minimum failure rate, and can be utilized for probabilistic risk analysis purposes.Copyright © 2006 by ASME